1. Field of the Invention
The present invention relates to a combined cycle plant comprising a heat exchanger which uses steam as a high temperature side working medium for recovering heat into compressed air or gas turbine fuel to be supplied into a gas turbine.
2. Description of the Prior Art
As a first prior art example for carrying out this type of heat recovery, a regenerative type gas turbine combined cycle plant will be described with reference to FIG. 4.
In a combined cycle plant formed using a regenerative type gas turbine shown in FIG. 4, a regenerative type gas turbine 01 constitutes a topping cycle of the combined cycle plant and comprises a compressor 1, a generator 2 connected to the compressor 1 via a shaft, a regenerator 3 for recovering heat into compressed air discharged from the compressor 1, a combustor 4 for burning fuel (supplied from outside the cycle plant) using heated air supplied through the regenerator 3, a turbine 5 operated by combustion gas sent from the combustor 4, an exhaust gas duct 6 for supplying therethrough exhaust gas from the outlet of the turbine 5 to a heating side of the regenerator 3, and an exhaust gas duct 7 for supplying therethrough the exhaust gas from the regenerator 3 to a waste heat recovery boiler 02.
In the waste heat recovery boiler 02 supplied with the exhaust gas from the turbine 5, heat recovery is done sequentially at a high pressure steam generator 29, an intermediate pressure steam generator 28 and a low pressure steam generator 27 to generate a saturated steam of high pressure, intermediate pressure and low pressure, respectively.
The high pressure saturated steam is led into a high pressure superheater 33 via a high pressure steam pipe 34 to be elevated in temperature to a predetermined level, and is then led into a high pressure turbine 22 of a steam turbine plant constituting a bottoming cycle of the combined cycle via a high pressure steam pipe 32 to expand there to generate power.
On the other hand, the intermediate pressure saturated steam is led through an intermediate pressure steam pipe 31 to be mixed on the way with steam from the outlet of the high pressure turbine 22 and to be elevated in temperature to a predetermined level at a reheater 26, and is then led into an intermediate pressure turbine 23 to expand there to generate power.
Also, the low pressure saturated steam is led through a low pressure steam pipe 30 to be mixed on the way with steam from the outlet of the intermediate pressure turbine 23, and is then led into a low pressure turbine 24 to expand there to generate power. The steam is thereafter condensed into water at a condenser 25 to be then supplied into the waste heat recovery boiler 02.
A second prior art example for carrying out this type of heat recovery will be described with reference to FIG. 5.
In a steam cooled type gas turbine shown in FIG. 5, the system is so constructed that steam from a high pressure turbine outlet is directly used for cooling of the gas turbine blades, and is then recovered into an intermediate pressure turbine. Meanwhile, air from compressor outlet is used for cooling of a combustor tail tube.
That is, in FIG. 5, numeral 01 designates a gas turbine, numeral 02 designates a waste heat recovery boiler, numeral 57 designates a high pressure turbine, numeral 58 designates an intermediate pressure turbine and numeral 59 designates a low pressure turbine. In the gas turbine 01, air taken into a compressor 55 is compressed to a predetermined pressure, and this compressed air from the compressor 55 is mixed with fuel for combustion at a combustor 56. In this process, the flow rate of the fuel is adjusted so as to attain a predetermined at the inlet of a turbine 54.
Combustion gas of high temperature and high pressure generated at the combustor 56 is expanded at the turbine 54 to work for power generation at a generator 70, and exhaust gas from the turbine 54 is supplied into the waste heat recovery boiler 02 via an exhaust gas duct 60.
High pressure exhaust steam from the outlet of the high pressure turbine 57 is supplied into the turbine 54 as cooling steam for cooling of the stationary blades and the moving blades thereof via a blade cooling steam supply pipe 61. The cooling steam heated through this cooling process is supplied into the inlet of the intermediate pressure turbine 58 via a blade cooling steam recovery pipe 62.
In the waste heat recovery boiler 02, high pressure steam generated at a high pressure drum 53 is led into the high pressure turbine 57 via a high pressure steam pipe 63 to expand there to generate power.
Outlet steam from the high pressure turbine 57 is bifurcated so that a first portion is led as blade cooling steam for the stationary blade and moving blade of the turbine 54 via the blade cooling steam pipe 61, as mentioned above, and so that a second portion is led into a reheater 74 of the waste heat recovery boiler 02.
Intermediate pressure steam generated at an intermediate pressure drum 52 is mixed with the second portion of the high pressure exhaust steam (which is the portion of the outlet steam from the high pressure turbine 57 to be supplied into the reheater 74) to be heated there and is then mixed with the blade cooling steam led through the blade cooling steam recovery pipe 62 to be supplied into the intermediate pressure turbine 58.
The steam so mixed and supplied into the intermediate pressure turbine 58 expands there to generate a predetermined power. The intermediate pressure exhaust steam, or the outlet steam from the intermediate pressure turbine 58, is mixed with low pressure steam generated at a low pressure drum 51 and supplied through a low pressure steam pipe 65 and is then supplied into the low pressure turbine 59 for generating a predetermined power.
Low pressure exhaust steam coming out of the low pressure turbine 59 is condensed into water at a condenser 71 to then be pressurized to a predetermined level at a pressure pump 72 to be fed into the waste heat recovery boiler 02 via a feed water pipe 73.
In the first prior art example of the combined cycle plant constructed as described above, the regenerative type gas turbine plant acting as the topping cycle of the combined cycle plant comprises the regenerator 3, as compared with a conventional simple gas turbine, so that the exhaust gas heat is recovered into the inlet of the combustor 4. Thus, the inlet temperature of the combustor 4 is elevated and an advantage is obtained of reducing the fuel flow rate and thus enhancing the gas turbine efficiency and the combined efficiency.
In order to obtain this advantage, however, it is necessary to provide a piping of the exhaust gas duct 6, which has a large size, from the outlet of the turbine 5 to the regenerator 3. It is also necessary to provide a piping of the exhaust gas duct 7, which is downstream thereof, from the regenerator 3 to the waste heat recovery boiler 02. Thus, the cost of the plant increases because the cost of the exhaust ducts 6, 7 is high.
Also, as the exhaust gas from the turbine 5 is first supplied into the regenerator 3 to pass therethrough, there occurs a large pressure loss of the exhaust gas. This reduces the turbine pressure ratio to hinder the original intent of the regenerative type gas turbine to enhance the turbine efficiency and the combined efficiency.
Furthermore, the heat exchange at the regenerator 3 is between the exhaust gas on the high temperature side and the compressed air on the low temperature side, and the heat transfer coefficient of the high temperature side heat transfer surface of the regenerator 3 is smaller as compared with the heat exchange with steam. As a result, the heat transfer area of the regenerator 3 becomes larger as compared with the heat exchanger of the waste heat recovery boiler in which the high temperature side is the exhaust gas and the low temperature side is the steam, which leads to a cost increase.
Also, in the second prior art example of the combined cycle plant, the blade cooling steam for cooling, the turbine stationary blade and moving blade of the steam cooled gas turbine is supplied directly from the high pressure exhaust steam coming out of the high pressure turbine 57. Therefore, if the inlet temperature or the inlet guide vane opening of the gas turbine or atmospheric air temperature or the like changes, the outlet temperature of the high pressure turbine 57 must change correspondingly. Thus, the temperature of the blade cooling steam also changes.
Change in the temperature of the blade cooling steam leads at the same time to a change in the temperature of the metal of the turbine blade or the like. Thus, particularly at the time of partial load, because the temperature of the blade cooling steam is prone to become high, there occurs a problem that the turbine blade is likely to cause a creep deformation.
As a general attempt to control the cooling steam supply temperature to cope with a tendency of high temperature of the cooling steam, it is considered to supply spray water into the cooling steam. However, in this case, in the process of the spray water being mixed with steam to be vaporized, impurities in the spray water come to be included in the cooling steam and stick to cooling passages of the steam cooled blade, which results in a possibility of corrosion of the steam cooled blade.
Also, in addition to the problem of corrosion, the impurities sticking to the cooling passages of the steam cooled blade reduces the heat transfer coefficient of the cooling passages, so that the temperature of the blade metal increases and there arises a possibility of creep deformation of the blade.
Corrosion of the cooling passages of the steam cooled blade makes the thickness of the blade thinner, so that cracks therein may be caused. In addition, there arises a possibility of leakage of the cooling steam into the turbine 54 of the gas turbine 01. Also, it is known that such leakage of the steam reduces the combined efficiency. Hence, it is an important matter to be avoided to have the impurities causing the corrosion mixed into the cooling steam.